Part of Russian Pres. Vladimir Putin’s state of the nation speech Thursday sounded like something from a 1960s James Bond film. Putin announced his country has developed and recently tested a cruise missile and an underwater drone that are nuclear-powered as well as hypersonic missiles capable of flying at up to 20 times the speed of sound. Putin’s words were punctuated by video and computer graphics the Russian leader used to drive home the point that the weapons would render NATO’s U.S.-led missile defense systems “useless.”

For the West, the prospect of Russia having all three of those new weapons is unsettling to say the least. But the nuclear-powered cruise missile, in particular, harkens back to a weapons system the U.S. Air Force began developing at the height of the cold war and later abandoned. Project Pluto (pdf), an initiative commissioned in 1957, had the goal of developing nuclear-powered engines for use in Supersonic Low-Altitude Missiles (SLAM). The Pentagon tested prototype engines with 500-megawatt reactors at the Department of Energy Nevada Test Site in 1961 and 1964, but soon after had second thoughts about the project.

Scientific American spoke with Edwin Lyman, a senior scientist in the Union of Concerned Scientists’ Global Security Program, about the eerie parallels between Putin’s nuclear-powered missile and Project Pluto—including the Pentagon’s reasons for ending the project, and the lessons about the dangers of nuclear power and weaponry that seem to have been forgotten.

[An edited transcript of the conversation follows.]

In what ways does Putin’s nuclear-powered cruise missile appear similar to Project Pluto?

Project Pluto was going to use a ramjet engine to draw in air at a supersonic speed, use a nuclear reactor to heat the air to get it to expand, and then use that expanded air to generate thrust. So the U.S. did contemplate a similar idea, the advantage being the use of long-lived nuclear fuel to keep the missile airborne for a long time, evade defenses, maneuver extensively and reach its target with a high level of accuracy.

Would the U.S. missile’s nuclear fuel also be part of its payload when striking its target?

Based on the plan for the U.S. program, the missile itself—even before reaching its target—would be a flying death factory [in addition to any nuclear payload that might be attached to the projectile]. The reactor would presumably not have any lead or concrete radiation shielding, because that would have made the missile too heavy. Such an unshielded missile would have generated a very intense flux of neutrons, so simply being in close proximity to it would have been lethal. The missile would have drawn air from the outside, heated it to a very high temperature with direct contact with nuclear fuel and then expelled the air. So fission products and radioactive particles would continuously be expelled into the environment.

How close was the U.S. Air Force to developing a working nuclear-powered missile?

They had a proof-of-principle reactor on the ground, but my impression is that at the time the project was canceled there was probably still a substantial amount of engineering work that needed to be done, not to mention flight testing. Amazingly, the prospect of testing this monster —even during the height of the cold war—was too horrific for the Air Force, which tells you something about the weapon they had in mind.

Was the Air Force concerned about the missile detonating during testing?

No, there were concerns about the hazards of operating this reactor in the first place. A brochure put out by the Nevada National Security Site [in 2013] points out that Pluto would “deafen, flatten and irradiate people along its flight path.” Clearly, in that era the Atomic Energy Commission and the Pentagon had shown themselves not to be overly concerned about civilian safety—they were still doing atmospheric testing of nuclear weapons—so their standards were pretty low for protecting the public. The fact that Pluto was too troubling even for them is a worrisome sign, given that Russia seems to have gone ahead with a similar project.

How long would it have taken Russia to create the weapon Putin described, assuming it is similar to the Project Pluto missile?

This may have been in response to the general deterioration of relations between the U.S. and Russia over the past decade and a half, dating back to the George W. Bush administration’s withdrawal from the Anti-Ballistic Missile Treaty in 2002. The U.S. and NATO’s continued development of ballistic missile defense has clearly been a concern for Russia, even though the U.S. and NATO have insisted those systems are intended for use against rogue states like Iran and North Korea rather than Russia. [Other steps in that direction were] the U.S. continuing to pursue a relatively strong posture in its deterrence strategy as well as the [2018] Nuclear Posture Review released by the Trump administration. Russia must have been working on this for some time if they have a successful system.

How likely is it that Russia has, in fact, developed the system that Putin described?

I can’t really say. I would note there was a Washington Post story today, citing Fox News, that says the Pentagon was aware of a Russian test of a nuclear-powered cruise missile—but the system was still under development and had recently crashed in the Arctic. You would think that a flight test of this type of weapon system couldn’t be concealed—so, someone knows. All I can say is that if it’s real, it’s a development of great concern and would show a level of recklessness in Russian decision-making that even the U.S. wasn’t going to engage in during the cold war.

Are the lessons learned decades ago about the devastation of nuclear weapons being ignored?

One had hoped that the 1950s dream of using nuclear energy to power everything had been put to bed. Putin’s new weapons and NASA’s talk of using nuclear reactors to power spacecraft suggest some of those dreams are being revisited. This Pandora’s box does seem to have been opened again on both sides, and all of the old objections as to why you would not want to put nuclear reactors on high-speed vehicles that you’d launch into space still apply. There seems to be a collective amnesia—policy makers and the public have to relearn lessons learned a long time ago.